Have you got a spare 2 million seconds of telescope time? Astronomers using the Hubble Space Telescope have! First we had the Hubble Deep Field (HDF) North and South, then the Ultra Deep Field (UDF) – and now we have the Hubble eXtreme Deep Field or XDF.

XDF – the Hubble eXtreme Deep Field. This image was assembled by combining 10 years of Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. The XDF is a small fraction of the angular diameter of the full Moon. (Credit: NASA, ESA, Illingworth, Magee, & Oesch (U.California, Santa Cruz), Bouwens (Leiden), and the HUDF09 Team)

The XDF is a centrally located sub-image of the UDF in the Fornax constellation. It is the deepest image of the universe ever taken. The new full-colour XDF image includes very deep exposures in near-infrared light, enabling new studies of the earliest galaxies in the Universe. Over 2,000 optical images from the HST Advanced Camera for Surveys Wide Field Camera and near-infrared images from Wide Field Camera 3/IR channel taken over the nearly ten year period of July 2002 to March 2012 were combined to produce the XDF image.

For those wanting more detail, ACS/WFC F435W(B)+F606W(V) filters were assigned to the blue channel, ACS/WFC F775W(I)+F814W(I)+F850LP(z) filters (green) and WFC3/IR F105W(Y)+F125W(J)+F160W(H) filters (red). The field is centred on RA 3h 32m 38s.5 and Dec.  -27° 47′ 00″ (in J2000 coordinates).

“The XDF is the deepest image of the sky ever obtained and reveals the faintest and most distant galaxies ever seen. XDF allows us to explore further back in time than ever before”, said Garth Illingworth of the University of California at Santa Cruz, principal investigator of the Hubble Ultra Deep Field 2009 (HUDF09) program.

The XDF contains about 5,500 galaxies even within its very small field of view of 2.3 by 2.0 arcminutes. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. In terms of magnitudes, that is a difference of 25. Hence if you can just see an object at mag 6 (typically the limit to which humans can see in the night sky) then XDF is able to detect objects at mag 31.

The addition of near-infrared imagery is important since distant galaxies will have their light greatly redshifted. Light emitted by young, hot stars in the ultra-violet region could well be detected by HST in the near-infrared from such distant objects.  Given that the
age of the universe is 13.7 Gyr, the XDF detects galaxies at lookback times of 13.2 Gyr. Hence the XDF is sampling very young galaxies, some only a few hundred million years old. Many of the galaxies detected are in the process of formation, some are colliding or merging with others, or rapidly forming stars.

Will it be possible to do more than just image the most distant galaxies in the XDF? The European Extremely Large Telescope (EELT) with a primary mirror of nearly 40m will collect about 16 times more light than a telescope with a 10m primary mirror. For the same instrumental set-up and exposure time, this gives the EELT the ability to observe about 3 magnitudes fainter than current 10m class telescopes. Hence in one night the EELT could get spectra of objects around mag 26 or 27. The faintest objects in the XDF at mag 31 could be imaged with the EELT, but spectroscopy of anything other than the brighter objects will be very difficult!

For more details, see

[Glen Mackie & Sarah Maddison]

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